Cold cathode gas-discharge tube



United States Patent COLD CATHODE GAS-DISCHARGE TUBE James M. Constable, White Plains, N. Y., assignor to Electronic Products Company, Mount Vernon, N. Y.

Application November 1, 1955, Serial No. 544,206 7 Claims. (Cl. 313-185) This invention relates to cold cathode gas-discharge tubes which may be used as pulse generators and the like to provide short-duration high-amplitude signals.

An object of this invention is to provide a two electrode cold cathode gas-discharge tube having a high breakdown voltage, a relatively low conducting voltage, and a very short de-ionization time.

Another object is to provide such a tube having improved stability and aging characteristics.

Signal generators, such as relaxation oscillators and the like, using cold cathode gas-discharge tubes are known in the art and are widely used because of their simplicity, efiiciency and low cost. In the past, however, such generators have usually been limited to relatively low-voltage, low-frequency applications since the tubes available had low-breakdown voltage and/or high ionization-deionization times. Accordingly these tubes are not at all suitable for use in pulse generators or circuits where pulses with very fast rise times, spacings of the order of one hundred microseconds and hundreds of volts amplitude are desired. In addition to this limitation, the electrical characteristics of many of these previously known tubes tended to vary over a period of time because of physical and chemical changes gradually taking place within the tubes. For this reason, the output signals of generators using such tubes could not be relied upon to remain constant and this further seriously limited their usefulness. The present invention seeks to overcome these difiiculties.

The breakdown voltage, the gas pressure, the ionization and de-ionization times of a gas-discharge tube are to a certain extent interrelated. It is known that a desired breakdown voltage for a given tube electrode spacing and configuration can be obtained by filling the tube with a selected gas at a predetermined pressure and, conversely, that by changing this pressure the breakdown voltage can be changed. However, there isa lower limit to the gas pressure which it is practical to use because, at very low pressures, absorption of gas molecules within the tube by the elements of the tube can of itself cause a great percentage change in the absolute pressure within the tube. This change in pressure, of course, causes a corresponding change in the breakdown voltage.

On the other hand, at gas pressures high enough so that gas absorption is no longer a problem, it is very difiicult to obtain a sufliciently high breakdown voltage with desirably short ionization or de-ionization times.

Complicating this problem still further is the fact that hydrogen gas, which frequently is used in an effort to obtain short de-ionization times, is very active and, therefore, when used in a gas tube, usually tends to cause chemical reactions and thus bring about appreciable alternations of the electrical characteristics of the tube over a period of time. Moreover, adsorption, absorption and dififusion of the hydrogen bring about the same effect.

In accordance with the present invention a gas discharge tube, suitable for use in pulse generators or circuits wherein sharp, closely spaced pulses are used, is

side wall of the anode.

2,833,951 Patented May 6, 1953 provided having coaxial inner and outer electrodes and having a moderate-pressure filling of hydrogen. Because of the advantageous arrangement and proportioning of the elements of the tube, together with the kind and pressure of the gas filling, a high breakdown voltage is achieved along with an extremely short ionization and de-ionization time. Moreover, the tube will retain these desirable electrical characteristics for an exceptionally long period of time even under severe operating conditions.

A fuller appreciation of the many advantages of the present invention, together with a better understanding of its general nature, will best be gained from a study of the following description given in connection with the accompanying drawings in which:

Figure 1 is a side elevation of a corporating the invention; and

Figure 2 is an enlarged cross-section of the electrode gas-discharge tube in structure of the tube shown in Figure 1 taken on line 2-2 therein.

Figure 1 shows a gas-discharge tube, generally indicated at 10, having a conductive cup base 12 and a conductive cap 14 both cemented to an evacuated and gasfilled glass envelope 16. Centered approximately within this envelope is an electrode structure, generally indicated at 11.

Figure 2 shows an enlarged cross-section of the electrode structure 11. This structure consists of two coaxial electrodes spaced apart by insulating cylindrical end spacers 18 and 19. Outer electrode 20, (which hereafter will be called cathode 20) because usually connected to a more negative, D. C. potential than inner electrode 22, (hereafter called anode 22) is a thin walled cylindrical tube of conductive material and is brazed or otherwise fastened to a pair of end leads 24 which are sealed through envelope 16 and connected to base 12. These leads 24, in addition to serving as an electrical connector, help support the lower end of the electrode structure. Anode 22 is a small diameter conductive cylinder with an opening along its axis which permits the insertion of a lead 26 is sealed through I cap 14, and also serves to support the upper end of electrode structure 11. Anode 22 fits snugly within recesses in spacers 18 and 19 and is supported by these spacers concentrically within cathode 20. The upper end of spacer 19 has a small opening 27, smaller than anode 22 and extending beyond, to provide a passage for lead 26.- Lead 26 is electrically connected to anode 22 by a spring hook 28 carried by the lead and tightly engaging the in- This hook can slide within the anode and flex slightly to take care of variations in dimensions caused by heating and cooling of the tube. A small opening 30 through the wall of cathode 20 (see Figure 1) is provided to equalize the gas pressure within envglope 16 between the regions inside and outside of catho e 20.

The total length of anode 22 and the depth of the recesses in spacers 18 and 19 are such that the shoulder 32 on spacer 18 and shoulder 33 on spacer 19 are just inside the ends of cathode 20. By bending in the ends of the cathode to engage these shoulders the electrode structure 11 can quickly and easily be fastened into an integral assembly.

Envelope 16 is filled to a moderate pressure, of the order of 15 mm. of mercury, with substantially pure hydrogen gas. This pressure is somewhat above the pressure which would give the lowest breakdown voltage for given electrode dimensions and configuration and Well above a pressure at which gas absorption by the tube elements becomes a problem. Thus, not only will the single electrical lead 26. This envelope 16 and connected to pressure of the filling stay substantially constant but, because the breakdown voltage is somewhat above the lowest breakdown voltage, this breakdown voltage-=willnot be substantially changed by small changes in pressure. The electrodes of the tube are dimensioned so that, with the above mentioned gas filling, a direct voltage of approximately 400 volts applied between thecathode and anode causes breakdown of the gas filling. With this breakdown voltage and with the kind and pressure of gas filling mentioned, a de-ionization time of tho'order' of 0.1 micro secondhas been obtained. By adding to the hydrogen from ten to twentypercent of its pressure of a noble gas, such as argon, the conducting voltage of the tube can be made even less.

In order to insure that the breakdown of the gas mole-' cules'into ionized particles, once started, rapidly proceeds to all portions within the region between cathode and aljtSidfi, therratio of the inside diameter of the cathode to the outside diameter of the anode is made equal to or slightly less than the number e, i. e. the base of the Napierian system of logarithms. With the gas pressure used in the tube, it is necessary to use a small insidecathode diameter so that a breakdown voltage of approximately 40 volts can'be obtained. Diameters of 0.340" and 0.125" respectively have been found satisfactory in one tube which has been built and tested. The effective length of each of theanode and the cathode of this tube is 0.375".

It has been discovered that, in a tube according to this invention'using a hydrogen filling, the following materials are advantageously employed to provide a high degree:

of stability to the electrical characteristics of the tube: an aluminum cathode and anode, chrome-iron electrode leads, ceranrioplugs, and a lead-glass envelope.

The above description is intended in illustration and.

9 ligiitafiiom Various changes and modifications in the embodiment described mayoccur to those skilled in the a rtancl these changes or modifications can be made without departingfrom; thespirit or scope of the inventionas set forth;

I claim:

cold cathode gas-discharge tube comprising coax-- means for supporting saidielectrodes in spaced relation th ai iav v e 2 A coldcathode gas discharge tubecomprising coaxne d ou er electrodes, having a ratio of insidediameter of the outer electrode torthe outside diameter of the inner, electrode approximately; equaktothe number 2, an envelope surrounding said electrode, hydrogen: gas

filling 'said envelope at a moderate pressure, ceramic plugs holding said electrodesfirmly in coaxiahrelation to each other, and chrome-iron leads. connectedno said electrodes and sealed through said envelope, said leadsservingalso to support said electrodes within, said envelope.

3, The Combination of elements as incIaim-Z in.which.

season 4 t each of said electrodes is a short aluminumtube and one of said leads is connected to said inner electrode by a flexible, slidable spring hook inserted within saidinner cathode, a pair of cathode conductors attached to said cathode and sealed through said envelope and passing to a base QQnncction on the outside thereof, said conductors serving to support one end of said cathode within said envelope, and an anode conductor sealed through said envelope passing through one of said annular plug spacers to within said anode and being attached thereto by a spring connection, said anode conductor serving to support the end of said anode which is opposite the endof said cathode supported by said cathode conductors.

5. A cold gas diode comprising a cylindrical anode having a diameter of the order of '45 inch, a concentric cathode surrounding said anode and having an inside diameter equal or slightly lessthan e times the diameter of said anode, a gas-tight envelope surrounding said anode and cathode, and hydrogen gas filling said envelope at a pressure just slightly higher than the pressure which would gi vea minimum breakdown voltagefor the spacing and configuration of said anode and cathode but low enough to give a de-ionization time of lessthan' 1 microsecond;

6. The combination of elements as in claim 5 in which saidhydrogen gas has a pressure of the order of 15 mm. of mercury and said cathode is spaced from said anode to give a breakdown voltage of the order of several hundred volts.

7". In a cold cathode gas tube an electrode structure comprising: a tubular inner'electrode; a tubular outer electrode; a pair of cylindrical insulating spacers fitted withinsaid outer electrode and having recesses into which said inner electrode is fitted and held concentrically within said outer electrode, one of said spacers having an opening along its axis and each of said spacers having a shoulder against which the corresponding end of said outer electrode is crimped thereby to fasten said electrodes and spacers into an integral assembly; a pair of electrical leads fastened to one end of said outer electrode; and an electrical lead passing through the opening in one. of" said spacers and making electrical contact with the inside of said inner electrode, said electrical leads serving tosupport saidelectrode structure within said tube.

V References-Cited'in the file of this patent UNITED STATES PATENTS 2,422,324 Watrous June 17, 1947 2,427,086 Arnott et al. Sept. 9, 1947 2,456,900 Sullivan et a1; Dec. 21, 1948 2,471,263 Depew May 24, 1949 

